Heretofore, it has been difficult to use secondary aluminum-silicon alloys in industry, because of the unsatisfactory properties of these alloys. It would be desirable to use recycled alloys in various fields, for example the automotive industry, agriculture (i.e. tractors) and aviation. Recycling is a preferred alternative in industry, offering economic advantage as well as better use of natural resources. Another important benefit of recycling is the preservation of the environment, since it uses waste metals which would otherwise contaminate the environment, and present a problem to dispose of. Throughout the present disclosure, the term "silumin" is used to indicate an Al--Si alloy.
One problem, however, is that recycling usually results in secondary alloys, i.e., alloys which include more than about 0.5% impurities, like Fe, Mg, Cu, Cr, Ni, Zn, Mn and/or others.
Likewise, there are lower grade ores which result in secondary alloys. In many cases, the impurities include an enhanced percentage of iron Fe (for example, up to about 0.7%).
A problem exists not only with secondary alloys. Even a smaller percentage of the above impurities in Al--Si alloys may have a detrimental influence on the properties of the alloy.
The low performance relates to the casting and mechanical properties of these alloys, for example their castability, porosity, machinability, ductility and fatigue strength.
The undesirable properties of these secondary alloys mainly result from the structure into which these alloys solidify, with the iron content crystallizing into specific structures which include, for example, a long needle morphology, or Chinese script, or needles. Structures including these morphologies with their undesirable properties appear while cooling the alloy, for example in a sand cast, at cooling rates between about 0.1 and about 1.0 degrees K/second. Similar structures with these morphologies also appear in other casting methods.
Several known methods have been suggested in an attempt to partly remedy these problems.
It has been suggested that the problem of large iron-bearing constituents might be remedied by:
1. increasing the cooling rate, PA1 2. reducing the iron content, PA1 3. adding elements which transform the iron constituents into a harmless shape, PA1 4. adding elements to the liquid, which dissolve the undesired phases into smaller parts. PA1 1. increasing the cooling rate, PA1 2. heat treating to dissolve or spherodize the compounds PA1 1. The eutectic changes from separated to coupled. PA1 2. There is a decrease in the surface tension of aluminum, that leads to silicon particles which are more rounded and smaller. PA1 1. L. F. Mondolfo, "Aluminum Alloys: Structure and Properties". 1979 p. 971. PA1 2. I. Minkoff, "Solidification and Cast Structure". 1986. John Wiley and Sons. PA1 3. Naeker, "Proceedings of the Conference on Thermal Analysis of Molten Aluminum" 1985 p.155. PA1 4. Zhao et al., "Effect of Zn on microstructure and properties of Al--Si alloy". Taiyuan Univ. of Technology/Journal of Special Casting & Nonferrous Alloys. 2 1994. pp. 5-7. Language: Chinese. The paper deals with the effect of Zn on the structure and properties of Al--Si alloy. Eutectic cell structure is formed in the modified alloy by the addition of certain Zn. PA1 5. Chichko et al., "On the parameters of nucleation of modified and nonmodified silumin metals". Belorusskaya Cosudarstvennaya Politekhnicheskaya Akademiya, Minsk, Belorus. Rasplavy n 5 September-October 1993, pp 83-86. Language: Russian. PA1 6. G. I. Eskin, "Ultrasonic Treatment of liquid Aluminum" Moscow, Metalurgia, 1982, pp. 232 (In Russian). PA1 7. I. G. Brodova, P. S. Popel, "The physics of Metals and Metallography:, Moscow, Metalurgia, 65, 21, 1988 (In Russian).
It has been suggested that the undesired influence of silicon impurities might be reduced by:
Increasing the cooling rate may be useful for both problems, but the required substantial increase in the cooling rate can only be achieved by changing to other casting methods, such as changing from sand casting to metallic mold casting, for example. These methods, however, tend to increase the internal tension in the casting, which may produce warping or cracking of the casting.
Metallic molds are very expensive, and are therefore not preferred over sand molds.
Moreover, changing to metallic molds requires a drastic change in the method of production, a costly alternative.
A reasonably efficient and effective known method is the addition of elements which transform the iron compound from relatively large plates or needles to smaller and less embrittling forms. The elements used, known as iron correctors, include manganese, chromium, nickel, cobalt, molybdenum and other elements.
These elements form compounds with the iron in the alloy, which crystallize into a phase with various forms like globular or dendritic forms, which do not have the undesired properties of the above-mentioned plates and needles.
Despite their deficiencies, in many cases the addition of iron correctors is chosen as a reasonable compromise.
These additional elements, however, add a significant amount to the cost of the alloy thus formed. Moreover, these additions influence the properties of the alloy. Thus, the percentage of iron-bearing compounds is increased to the point that they influence the solidification mode and reduce the fluidity of the alloy.
There is a complication in the manufacturing process, since the preparation of a master alloy is required. This makes the process more expensive. Master alloys are required since aluminum will not accept certain materials, molybdenum for instance.
Machinability, too, is reduced, especially if primary crystals of the compounds are formed.
As known in the art, the silicon phase can be present in several structures. The eutectic can be random, nonmodified, undermodified, modified and overmodified. The primary silicon crystals can appear as globular or plate-like shapes, as well as feathery, star shaped or spherodized.
If the alloy contains more than about 0.8% Fe, then primary Fe Si Al.sub.5 crystals appear.
If Mn is also present in the alloy, then the compound (Fe Mn).sub.3 Si.sub.2 Al.sub.15 is formed. This compound has the shape of Chinese script, thus the embrittling effect of Fe Si Al.sub.5 is eliminated.
One known method of refining microstructure grains and precipitates is ultrasonic treatment of liquid metals. Treating a metallic melt with ultrasound results in transition from dendrite to non dendrite structure.
If the total content of manganese plus iron in the alloy exceeds about 0.8%, then the (Fe Mn).sub.3 Si.sub.2 Al.sub.15 crystals are primary and they appear as hexagonal globules. These globules do not embrittle the alloy, but they reduce its machinability.
If the Cr or Ni are present in the alloy, the compounds (CrFeMn).sub.x Si.sub.y Al.sub.z or (NiFeMn).sub.x Si.sub.y Al.sub.x are formed, respectively.
A preferred method for decreasing the mean size of the iron-bearing precipitates of the eutectic origin includes the modification of the alloy structure by the introduction of small amounts of specific elements (modifiers). The idea is to achieve a more disperse structure of the eutectics and, as a result, to decrease the size of the eutectic iron-bearing inclusions which are comparable in size with eutectic phases.
All of the alkaline and most of the alkaline earth metals achieve the modification effect.
The most used metal is sodium, which is also the cheapest. Strontium is also widely used. The other alkaline earth metals are less effective. To achieve the desired modification effect, the percentage of the sodium addition should preferably be about 0.01-0.02%.
Because of the limited miscibility and the strong tendency of sodium to oxidize, however, larger amounts of sodium are added, especially if the melt is not poured immediately. Sodium is a difficult metal to handle. It tends to float on the melt and is preferably kept immersed until melted; it oxidizes rapidly and its effect disappears in a short time.
It is known that sodium decreases the grain size of the alloy. This is a desirable effect. Sodium, however, has an adverse effect on the castability of the melt. Sodium has no influence on the iron phase, and therefore is more useful for relatively clean alloys.
Modification can also be produced with alkaline metal salts if they decompose in contact with the melt, but the salts which are effective are very expensive and are not too efficient.
The elements which nucleate the silicon and distribute the primary silicon crystals include: arsenic, sulfur, selenium, tellurium and gallium plus tellurium. Boron together with titanium refines the grain size of the aluminum but does not appreciably affect the silicon appearance.
Small amounts of alkaline or alkaline earth metals or alkaline metal salts change drastically the appearance of the silicon crystals, which become smaller, more rounded and form a coupled eutectic. Basically, there are two processes which occur:
Unfortunately, there is no transformation of the iron-bearing constituents, which retain their undesired shape.
One of the most efficient methods to decrease the volume fraction and the main size of the primary iron-bearing precipitates is sufficient overheating of the silumin melt, above liquidus. It was found that liquid Al--Si alloys conserve microheterogeneous state for a long duration after the ingot melting or components mixing at a temperature above liquidus. The microheterogeneity is inherited from the initial heterogeneous property of the material.
Just after their melting, fragments of various solid phases begin to dissolve. However, the dissolving process is not completed immediately, and no true solution is formed in the initial stage.
During a first stage, the melt has a colloidal structure, comprising an aluminum-bearing solvent and disperse (on the order of about 10 nm size) colloidal particles including silicon, iron and other elements. These particles either dissolve very slowly or remain in a metastable state of equilibrium with the surrounding melt.
In any case, at a temperature slightly above liquidus, the system conserves its microheterogeneity for a period on the order of about 10 hours, that is, during the whole melting process. When the melt is crystallized, the above-mentioned colloidal particles become the nuclei of solid silicon, iron and other element bearing phases. This may result in an alloy with inferior performance, as detailed above.
Further details regarding the above-described processes may be found, for example, in the following literature:
Cooling curves are built for non-inoculated and Na-inoculated aluminum-11.5-12.5% silicon alloys. The nuclei number and growth rate are estimated as a function of temperature (35-125 degrees C.) for each curve by the program developed. The Kolmogorov model corrected for the heat balance equation.
Prior art patents which may have a relationship to the present invention include the following:
Langenbeck et al., U.S. Pat. No. 4,799,978, details an aluminum powder alloy having good high temperature performance--contains iron, nickel and chromium. Hot worked Al alloy powder article consists of (in %): al 81-91.9 esp. 86, Ni 4-8 esp. 6, Fe 4-8 esp. t and Cr 0.1-3 esp. 2. USE/ADVANTAGE--Especially in manufacture of aircraft parts exposed to elevated temperatures. Alloy retains its mechanical properties even after prolonged exposure to tempertures up to 800 deg. F.
Mahajan et al., U.S. Pat. No. 4,787,943, details a dispersion strengthened aluminum alloy--contains titanium and rare earth(s). Al alloy dispersion-strengthened with rare earth metal(s) comprises (in wt. %): Ti 2-6, rare earth(s) 3-11, (VIII) element(s) 3 max and Al the balance. Pref. rare earth is Gd. Pref. max. at ratio of Ti to rare earth is about 2:1. The amount of (VIII) element is 0.1-3.0 wt. % and the preferred element is Fe. A specific alloy has the composition Al-4Ti-4Gd. In a typical process, 75 micron thick ribbon is formed by casting onto a chill wheel and annealing at 100-600 deg.C. for about 1 hr.
Schuster et al., WO 8706624, U.S. Pat. No. 4,786,467, details a cast metallic matrix with refractory reinforcement with good stiffness obtained by mixing melt without gas entrainment using shearing action. In the production of a composite material comprising an alloy reinforced with particulate non-metallic refractory the latter is mixed with the molten matrix under such conditions that introduction and retention of gas are minimized, and that the particles do not degrade in the mixing time. In mixing the particles and melt are sheared past each other to promote wetting. The mixture is then cast at such a temperature that no solid metal is present. Preferably the matrix phase is an aluminum alloy and the reinforcing phase is silicon carbide, alumina, boron carbide, boron nitride or silicon nitride.
Kubo et al., EP 241198, JP 62240727, U.S. Pat. No. 4,789,605, details a light metal matrix composite material with good high temperature properties--having reinforcing phase including potassium titanate whiskers. A composite material comprises a matrix of light metal reinforced with a mixture of potassium titanate whiskers and short fibre material selected from silicon carbide or nitride whiskers, alumina short fibers, crystalline or amorphous alumina-silica short fibers. The overall proportion of reinforcing phase is 5 to 50 vol. %, and the proportion of titanate whiskers in that phase is 10 to 80 vol. %. Preferably the proportion of reinforcing phase is 10-40% and the proportion of titanate in it is 20-60%.
None of the above-mentioned patents or literature appear to disclose or suggest an approach comprising both modification and overheating. The inventors are aware of patents using sodium, but are not aware of any patent which discloses or suggests the use of strontium.
Recycling of metals is highly desirable in the automotive industry, as well as in agriculture (i.e. tractors) and aviation. A problem with recycling is that it usually results in secondary aluminum-silicon alloys, that is alloys which include impurities such as Fe, Mg, Cu, Zn, Cr, Mn and/or Ni and others. These secondary alloys have unsatisfactory properties, for example mechanical properties and poor castability, as well as unsatisfactory heat resistance and other properties.
These unsatisfactory properties mainly stem from the structure into which these alloys solidify, for example Fe precipitating into long needles, plates or skeleton morphology shapes (about 100 micrometers long). Prior art solutions to the iron-rich precipitates include the addition of elements known as "iron correctors". These additives include manganese, chromium, nickel, cobalt, molybdenum and others. They form compounds with the iron in the alloy, which crystallize into various forms that do not have the above-mentioned undesired properties of the prior plate or needle structures. For example, if Mn is introduced into the alloy, then the compound (FeMn).sub.x Si.sub.y Al.sub.z is formed, which compound has the shape of Chinese script (skeletons), thus eliminating the embrittling effect of FeSiAl.sub.5.
The above additional elements, however, significantly increase the cost of the alloy. Moreover, other properties are detrimentally affected, including a reduction in the fluidity of the alloy, and a reduction in machinability. It is known that sodium is a good modifier, however it has a detrimental effect on castability.